There currently exists in the vast field of performance plastics a need for a low cost polyester material which can be easily molded or otherwise formed into various articles requiring a superior combination of high impact strength, high hardness, and high heat resistance. These properties would be especially desirable in melt processable polyesters used for injection molded plastics, fibers, film and sheeting.
The performance plastics utilized in industry generally have 1) high impact strength with a notched Izod strength of at least 1 ft-lb/in (53.4 J/m), according to ASTM D256; 2) high hardness, such as a material having a Rockwell L hardness of at least 70, according to ASTM D785; and 3) high heat resistance, such as a material having a heat deflection temperature of at least 70.degree. C. at 264 psi (18.6 Kg-force/cm.sup.2) loading, according to ASTM D648. These industry standards, herein, are used to define the terms "high impact strength," "high hardness," and "high heat resistance." There are no known copolyesters having a combination of all three of high impact strength, high hardness and high heat resistance.
Commonly used melt processable polyesters in the performance plastics industry are copolymers made from terephthalic acid, 1,4-cyclohexanedimethanol, and ethylene glycol. These copolymers have been the best available melt processable polyesters for most uses due to the fact that they provide for good machinability during processing, provide high impact strength and have relatively low cost. However, these currently used copolyesters are often deficient in that they suffer from low heat resistance and only moderate hardness.
Many articles which are currently formed from copolyesters from terephthalic acid, 1,4-cyclohexanedimethanol and ethylene glycol, referred to hereinafter as the "PET/CHDML copolyester" for convenience, would greatly benefit from increased hardness and/or heat resistance. Examples of articles formed from PET/CHDML copolyesters having hardness and heat resistance include molded polyester exterior panels used in automobiles and farming equipment, small molded polyester appliance parts, and exterior and interior glazing sheets used to replace windows and glass.
It is generally known in the polyester industry that the glycol component of the PET/CHDML copolyester must contain a substantial amount of 1,4-cyclohexanedimethanol to provide high impact strength.
Ethylene glycol is the main diol modifier in the known PET/CHDML copolyesters. The glycol component of PET/CHDML copolyesters is modified with ethylene glycol in order to strike a balance between the stiffness of polyethylene terephthalate (PET) homopolyesters and the toughness of 1,4-cyclohexanedimethanol polyesters. It is known that increasing the mole percentage of ethylene glycol present in PET/CHDML copolyesters increases the hardness and decreases impact strength. However, high hardness, as defined in this application, can still not be achieved in this system.
Existing copolyesters having high hardness levels and high heat resistance tend to be prepared from all-aromatic components. The rigidity and relatively high glass transition temperatures, T.sub.g, of the aromatic constituents provide for more hardness and heat resistance. However, those same properties also tend to make the copolyesters more brittle, providing insufficient impact strength.
Copolyesters based on terephthalic acid and 2,2,4,4-tetramethyl-1,3-cyclobutanediol having high impact strength are disclosed in U.S. Pat. No. 3,313,777. This patent discloses polyesters based on 2,2,4,4-tetraalkyl-1,3-cyclobutanediols which have resistance to oxidative and hydrolytic degradation and provide a copolyester having superior weathering characteristics and moisture resistance.
In light of the above, it would be very desirable in the performance plastics industry to have an economical copolyester having excellent machinability and a superior combination of high hardness, high heat resistance, and high impact strength.